Methacrylic resins made from methyl methacrylate as a principal component have excellent weatherproof and optical properties, and relatively balanced mechanical, thermal and fabrication properties. The methacrylic resins are used in various fields, such as automobile parts, electrical equipment parts, face plates, signs, covers for lights, ornaments, and variety goods, because of their properties mentioned above.
However, the methacrylic resins are insufficient in terms of heat resistance, and therefore their use in the fields requiring shape stability at high temperatures is limited. Thus, there is a strong demand for an improvement of the heat resistance of the methacrylic resins.
Many methods have been proposed for improving the heat resistance of the methacrylic resins. Examples of the proposed methods include the copolymerization of methyl methacrylate and N-aryl maleimide (Japanese Publication for Examined Patent Application, No. 9753/1968), and blending the copolymer of methyl methacrylate, .alpha.-methyl styrene and maleic anhydride with a methyl methacrylate copolymer (Japanese Publication for Unexamined Patent Application, No. 122536/1984).
Moreover, as known methods for producing transparent methacrylic resins of improved heat resistance and reduced coloring factor, there are methods including copolymerizing methyl methacrylate and N-cyclohexylmaleimide within a specific range (Japanese Publication for Unexamined Patent Application, No. 156115/1987 and 177009/1987), and a method including copolymerizing methyl methacrylate and N-cyclohexyl maleimide and then reducing raw material monomers remaining in the resultant copolymer (Japanese Publication for Unexamined Patent Application, No. 112612/1987).
By copolymerizing a methacrylic ester such as methyl methacrylate and N-substituted maleimide such as N-cyclohexylmaleimide, it is possible to achieve some improvements in respect of the heat resistance, transparency, and coloring factor, thereby providing a transparent heat-resistant resin for optical uses.
However, in such a heat-resistant resin, the improvement of the heat resistance depends on the amount of N-substituted maleimide. Therefore, in order to obtain a heat-resistant resin of improved heat-resistance, it is necessary to increase the N-substituted maleimide unit in the heat-resistant resin. However, the increase of the N-substituted maleimide unit causes a problem that the resultant heat-resistant resin tends to color (yellow).
In the case of N-substituted maleimide-based heat-resistant resins produced from N-substituted maleimide as one of the raw materials, the coloration of resin products is generally known. However, some uses of the resin products strongly require a reduction in the coloration of the resin products. Thus, there has been a demand for the development of N-substituted maleimide-based heat-resistant resins of reduced coloration (referred to as "low coloring factor" in the present invention).
In the production of molded articles by molding the heat-resistant resins obtained by the above-mentioned methods, the molded articles sometimes had lowered surface smoothness, and showed deterioration in their optical properties including transparency, and appearance.
The present inventor studied to find the cause of the lowering of the smoothness, and discovered that non-polymerizable impurities derived from the raw material in producing the N-substituted maleimide were the causes of such a lowering of the smoothness.
Since most of such impurities have their melting points at temperatures lower than 200.degree. C., when a heat-resistant resin containing the impurities is molded, the impurities volatilize due to heat in molding, and adhere to the mold surfaces. As a result, the smoothness of the mold surfaces is impaired. The impairing of the smoothness of the mold surfaces deteriorates the smoothness of the surface of the molded article obtained by the molding, and worsens the appearance and transparency of the molded article.
It was also discovered by the present inventor that such impurities adhere to an air pressure-reducing line and adhere as gum to an outlet of the heat-resistant resin in the process of producing a heat-resistant resin by copolymerizing the N-substituted maleimide and a monomer components containing, for example, methacrylic ester as a principal component, for example, in the solvent removing step of removing a solvent, etc. under conditions of heat and reduced pressure by an extruder, a flash tank, etc. after solution polymerization. Such adhesion of impurities may prevent stable production of heat-resistant resins.
Examples of the impurities are non-radical polymerizable compounds having a functional group, such as carboxyl group, acid anhydride group, primary amino group and secondary amino group. According to Japanese Publication for Unexamined Patent Applications, No. 221974/1993, 201972/1993, and 135931/1994, when the resultant N-substituted maleimide is stored, these impurities cause a change in the color of the N-substituted maleimide with time. These publications also disclose the inclusion of another purifying step like water treatment in the production steps as a method for reducing the impurities.
However, the conventional method disclosed in the above-mentioned publications can not easily eliminate disadvantages of the resultant heat-resistant resin and the production thereof which are caused by the impurities. The reason for such disadvantages is that this method additionally requires a special purifying step like water treatment in order to reduce the impurities, or has difficulty in reducing the impurities depending on the type of N-substituted maleimide.
In the case where an increase in the heat-resistance of the resultant heat-resistant resin is required, it is necessary to increase the amount of N-substituted maleimide units. However, if a monomer mixture containing a large amount of N-substituted maleimide is polymerized, the heat-resistant resin tends to color during polymerization and heating in the molding process. Consequently, it is difficult to obtain heat-resistant resins of low coloring factors.
Therefore, the use of antioxidant such as a phosphorus-based antioxidant and a phenol-based antioxidant was proposed as a method for reducing the coloring factor of a heat-resistant resin containing a large amount of N-substituted maleimide units in heating.
For example, Japanese Publication for Unexamined Patent Application, No. 256551/1989 discloses a method for producing a styrene-methyl methacrylate-based resin composition by mixing a thioether-based organic sulfur stabilizer and a phosphite compound with a styrene-methyl methacrylate-based resin obtained by polymerizing monomer components containing methyl methacrylate, styrene and a maleimide compound, and further mixing a bisphenol-based antioxidant with the styrene-methyl methacrylate-based resin, if desired, for the purpose of improving the thermal stability.
On the other hand, Japanese Publication for Unexamined Patent Application, No. 304045/1988 discloses the addition of an organic phosphorus-based compound to a methacrylate-based polymer obtained by copolymerizing methyl methacrylate and a maleimide compound as a method for producing a methacrylate-based resin composition of improved heat decomposition resistance. Additionally, this publication discloses the addition of an organic sulfur-based compound to the methacrylate-based polymer, and the addition of an organic sulfur-based compound as a chain transfer agent during polymerization based on the above-mentioned method.
Japanese Publication for Unexamined Patent Application, No. 116331/1994 discloses a method for producing a transparent heat-resistance resin by polymerizing a composition formed by adding a phosphorus-based compound to a monomer mixture containing N-substituted maleimide for the purpose of reducing yellowing in the heating step.
Thus, the above-mentioned publications describe the use of antioxidants, such as phosphorous-based compound and phenol-based compound, to achieve improvements in respect of the thermal stability and coloring factor.
However, in the methods disclosed in Japanese Publication for Unexamined Patent Application, No. 256551/1989 and 304045/1988 above, since the antioxidant is not added during polymerization, i.e., the antioxidant is all added after the completion of the polymerization, the effect on a coloring source produced during the polymerization may not be fully exhibited.
Furthermore, in the method disclosed in Japanese Publication for Unexamined Patent Application, No. 116331/1994 above, since the antioxidant is all added to the monomer mixture and then cast polymerization is performed, the polymerization of the monomer mixture is interfered by the antioxidant, and the conversion rate of the monomers to polymer may be lowered. Namely, the methods disclosed in the above-mentioned publications suffer from the problems that the efficiency of producing heat-resistant resins is low, and the heat-resistant resins tend to color more easily.
In the method disclosed in Japanese Publication for Unexamined Patent Application, No. 304045/1988, even when the organic sulfur-based compound as the chain transfer agent is added during polymerization, a sufficient coloration reducing effect is not produced in the polymerization.
In addition, for example, the following publications propose a method for reducing the coloration of heat-resistant resins containing N-substituted maleimide units during polymerization and molding. Japanese Publication for Unexamined Patent Application, No. 252211/1986 proposes reducing the residual maleimide by washing polymer beads obtained by suspension polymerization with an alcohol having 1 to 4 carbons. Japanese Publication for Unexamined Patent Application, No. 304013/1988 proposes polymerization using a specific initiator. Japanese Publication for Unexamined Patent Application, No. 310853/1993 proposes performing cast polymerization by adding a benzyl alcohol or substituted benzyl alcohol to the monomer mixture.
However, these conventional methods suffer from the problems that the number of production steps is increased, and the coloration reducing effect is not sufficient. The method disclosed in Japanese Publication for Unexamined Patent Application, No. 310853/1993 is cast polymerization, and alcohol is not removed. In this method, if a large quantity of alcohol is used, a lowering of the heat resistance and the expansion of the heat-resistant resin in heating tend to occur due to the alcohol remaining in the resin. Thus, there is a possibility of impairing the appearance. Consequently, in this method, the amount of alcohol to be added is limited to a small amount not greater than 1 percent by weight of the total amount of monomers, for example, 0.5 percent by weight. As a result, a sufficient coloration reducing effect is not exhibited.
As a method for reducing the coloration of heat-resistant resins containing N-substituted maleimide units during polymerization or molding, Japanese Publication for Unexamined Patent Application No. 234063/1987 discloses heat stabilization of N-cyclohexyl maleimide by adding maleamic acid, etc. to the N-cyclohexyl maleimide. In this method, suspension copolymerization of N-cyclohexyl maleimide and vinyl chloride is performed to produce a copolymer of a reduced coloring factor.
However, in this conventional method, since the N-cyclohexyl maleimide to which the maleamic acid is added is copolymerized, a radical polymerization (polymerization reaction) of radical polymerizable maleamic acid and monomers of N-cyclohexyl maleimide, etc. proceeds. This may impair the transparency and heat-resistance of the resultant heat-resistant resin.
It is an object of the present invention to provide a raw material used for producing heat-resistant resins of a low coloring factor, more particularly an N-substituted maleimide-based heat-resistant resin having excellent heat resistance, fine appearance including transparency, and a lower coloring factor.
It is another object of the present invention to provide a heat-resistant resin produced from the above-mentioned raw material, more particularly an N-substituted maleimide-based heat-resistant resin having excellent heat resistance, fine appearance including transparency, and a lower coloring factor, and to provide a process for producing such a heat-resistant resin.
It is still another object of the present invention to provide a process for stably and simply producing a heat-resistant resin having excellent transparency, heat resistance, and a low coloring factor.